In modern wireless communication systems like Long Term Evolution (LTE), the base station determines the used transport format, transport block size, MCS scheme, MIMO transmission mode, etc for both downlink (DL) and uplink (UL). To be able to do this determination for the DL the base station requires information from the terminal regarding properties of the current DL channel, commonly referred to as Channel State Information (CSI). In LTE Release-8, CSI consist of Channel Quality Information (CQI), Precoder Matrix Index (PMI), and Rank Indicator (RI). In later releases other forms of CSI may also be possible such as explicit channel quantization, effective channel quantization including receiver processing, noise plus interference feedback as well as receive covariance feedback. To enable such measurement reports the terminal measures on DL Reference Signals (RS). In LTE Release-8 the used RS are cell specific and transmitted frequently.
In LTE-Advanced i.e. Release-10 and beyond, cell specific RS used for CSI estimation are less frequently transmitted since demodulation of downlink transmission in many transmission modes is supposedly based on UE specific RS. However, determination of CSI is still based on cell specific RS—which are now less frequently transmitted to maintain or even reduce the total RS overhead.
Since the cell specific RS used for CSI measurements are sent less frequently, the UEs have a reduced amount of cell specific RS to choose from when they are requested to measure the CSI. This results in that many UEs will send the CSI reports in the same subframe risking congestion in the uplink.